In this renewal period, we shall use quantum mechanical methods, molecular dynamics simulations and free energy perturbation methods to continue to probe the relationship between structure and function of two families of ubiquitous metabolizing heme proteins, peroxidases and the cytochrome P450s. The specific studies that will be made include (1) Continued characterization of the steps in the transformation of these enzymes to their catallytically active form (2) Continued Studies of Substrate enzyme complexes for prediction of product selectivity by specific substrates by these two types heme proteins (3) Elucidation of role of bound water, protein architecture and specific point mutations of P450 isozymes in modulating enzyme function; (4) Calculations of the relative free energies of binding inhibitors to P450cam and (5) Characterization of the substrate binding sites of two horseradish peroxidases